1. Field of the Invention
The invention applies to satellite communications and more particularly to a broadband access system using a constellation of satellites in low Earth orbit to provide broadband services, essentially multimedia services.
The invention applies to access systems using geostationary satellites (GEO) and also to access systems using satellites in low Earth orbit (LEO).
2. Description of the Prior Art
The invention applies very particularly to a new broadband access system that is currently under development. This system is called SkyBridge. It makes available to users worldwide services such as fast access to the Internet and videoconferencing. It will use a constellation of 80 LEO satellites which link business and domestic users equipped with low-cost terminals to terrestrial gateways.
The SkyBridge system has two segments:
A space segment: the space segment comprises 80 LEO satellites (plus spare satellites) in orbit at an altitude of 1,457 km and the ground control segment comprising the satellite control center and tracking telemetry and control stations. The space segment provides permanent coverage in the band of latitudes from +68xc2x0 to xe2x88x9268xc2x0. It connects each SkyBridge user to the nearest gateway.
A terrestrial segment: the terrestrial segment comprises the terrestrial gateways and the user terminals. The former provide interconnection via an ATM switch with local servers and with broadband and narrowband terrestrial networks.
The SkyBridge access system is based on the asynchronous transfer mode (ATM) used to connect users to a local switch. Traffic emanating from terminals is transmitted in a transparent manner by the satellite (i.e. without any processing other than amplification and frequency conversion) to the gateway and vice versa. The gateways have switching functions and serve as interfaces with terrestrial networks.
The Earth is divided geographically into areas with a radius of about 350 km which comprise up to 2,000,000 potential clients and a gateway between those clients and the satellites of the constellation which illuminate the area. The station enables clients of the service in that area to communicate with the constellation.
The satellites of a low Earth orbit constellation generally move across the sky and can illuminate a geographical area z defined in the above manner for at least a few minutes and at most around twenty minutes.
The access system must nevertheless provide a certain bit rate between the constellation and the fixed points that the gateways form in this environment.
To this end, it is necessary to optimize the resources available on board each satellite. A way must be found to allocate the resources to points on the ground so that the services offered can be provided and meet the demand from clients.
The resources for a satellite are the number of antennas or individual beams multiplied by the number of channels (frequency resource) available to each satellite in the frequency band reserved to the access system.
One way of increasing the number of frequency bands that can be used in the band reserved for the system is to transmit with one or other polarization (which in practice doubles the number of frequency bands in the band reserved to the system). However, this still means that the same channel must not be used for neighboring areas.
However, it is then necessary to solve the problem of interference between waves when using the same frequency band for adjoining areas and to prevent any interference. Using the same frequency band for neighboring areas (i.e. adjacent or closely spaced areas) causes interference which reduces the signal to noise ratio of the received wave.
In the new SkyBridge system, the antennas can be steered. They will be mobile antennas whose orientation is controlled mechanically.
Each satellite illuminates a region with a radius of 3,000 km. A satellite can produce up to 45 individual beams simultaneously.
As the satellites move, the gateways on the ground must switch traffic to a new satellite in a manner that is transparent for users.
Once a satellite-ground station link has been set up, it is maintained throughout the time for which the area in which the ground station is located is visible from the satellite.
Ground allocation plans established for a given period must take into account all dynamic aspects of the new system and provide a good grade of service. To provide this grade of service to the network operator it is preferable to maintain the same link for as long as possible. This is because each change of link, although transparent for the users, necessitates synchronization signals between the connection satellite and the user. These communications are synchronization signals and not payload information (user communication), and result in a reduction of bandwidth for the network operator.
The present invention solves this problem of the continuity of the links.
The present invention makes it possible to provide a good grade of service to the network operator and to the end user by increasing the duration of a link while satisfying all demands, despite all the dynamic aspects of the system associated with the fact that the antennas can be steered.
The present invention therefore consists in a method of allocating links between a set of areas each equipped with at least one gateway and a set of satellites at successive times, the following steps being iterated each time:
detecting links which must be interrupted among all links allocated the previous time using a constraints propagation algorithm, and retaining the other links in a new allocation plan, and
using an optimization algorithm to allocate the remaining links, conforming to conditions imposed by interference problems.
In accordance with another feature of the invention, the constraints propagation algorithm used for detection comprises the following steps:
fixing at time T the allocated links of the previous allocation plan established at time Txe2x88x921,
eliminating the impossible links, defined by the known input variables for the new plan at time T,
determining all remaining areas for which the demand is not satisfied.
In accordance with another feature of the invention, the optimization algorithm for allocating links between all the remaining areas and all the satellites illuminating those areas, the links being determined by a channel, includes the following steps, which are iterated for each channel and for each satellite:
constructing an interference graph,
use of the graph by a search algorithm associated with a constraints propagation algorithm to verify the capacities.
The construction of the graph at time during which a satellite covers a number of areas and has a number of beams and a number of channels includes the following steps:
constructing the graph for that satellite in which each node of the graph corresponds to an area such that the number of channels demanded for that area is greater than the number of channels allocated,
updating the graph by removing or adding an area as and when links are allocated.
In accordance with another feature of the invention, the search algorithm utilizes the following steps:
calculating an interference coefficient for each pair of areas of the graph for a channel,
comparing the sum of the interference coefficients for each area to a predetermined maximum acceptable interference threshold,
creating a partition of at most the same number of elements as the number of beams which includes areas such that for each area the sum of the interference coefficients does not exceed the predetermined threshold.
The iteration for each channel and for each satellite consists in reiterating all the steps of the method for another channel up to the last channel and then reiterating all of the preceding steps for another satellite up to the last satellite.
Other features and advantages of the invention will become apparent on reading the following description, which is given by way of non-limiting example and with reference to the drawings.